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I'm using Superduper to make a copy of my system disk to an external conventional harddisk (HDD, not SSD). Superduper wants to convert the disk to APFS. Until now I've used HFS+. Is there a downside on using APFS? I know the advantages of APFS are mostly lost on a HDD, and that older systems don't work with APFS, but for me that is no issue.

Are there downsides on using APFS that I should know of?

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Yes there are downsides to HDD formatted APFS instead of HFS+

The newer format is optimized for solid state characteristics.

The older format is optimized for HDD characteristics.

Unless you are sure you need the new features, stick with the format that’s best for your storage type. If you don’t care about long term performance or use the storage lightly, it can be fine and not even measurable but heavy use will cause you to wipe and reformat the HFS in my experience if you choose and use APFS for months of continuous use on HDD as the inefficiencies build up over time and the storage allocation cannot self-optimize like HFS does on HDD.

My advice: Use Time Machine if you only have HDD to back up or provide SSD storage to Super Duper.

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  • I use two 2.5" external harddisks (HDD) for backup. Both have two partitions, one small partition for an exact copy of the system disk, the other for Timemachine. It's a backup that I rarely use, but I have used it when an update corrupted the system. I'm going to try it on one disk, and see from there.
    – SPRBRN
    Commented Aug 15, 2020 at 14:45
  • @SPRBRN It will work, just won’t be fast with APFS. I look forward to your experiments and also glad you’re backing up fast or slow.
    – bmike
    Commented Aug 15, 2020 at 14:53
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    From looking at the linked page, the performance issues are mainly noticeable when booting from a HDD or when enumarting files (so, as mentioned on the page "If performance of your rescue backup is of paramount importance, then you should consider replacing your HDD backup disk with an SSD"). So using APFS as a target for an SD! backup might be fine as long as one doesn't expect to boot from it later.
    – nohillside
    Commented Aug 15, 2020 at 15:38
  • @nohillside this is precisely my experience. As long as I don’t need hot file optimization, automatic file defragmentation and have less than 20 snapshots, APFS is far more modern for my tastes. However, I can’t seem to not enjoy snapshots and other features and HDD eventually need to be erased in my experience with APFS as performance degrades with use.
    – bmike
    Commented Aug 15, 2020 at 15:59
  • @nohillside - thanks! One question... what is enumeration?
    – SPRBRN
    Commented Aug 16, 2020 at 15:08

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